The present invention relates to a digital-to-analog converter circuit, and more particularly, to a digital-to-analog converter circuit for PCM (Pulse Code Modulation)-related equipment.
FIG. 1 exemplifies a conventional digital-to-analog circuit for use in digital audio disc players. In FIG. 1, the digital signal obtained from the disc is supplied to a digital-to-analog converter 1. The output terminal of the digital-to-analog converter 1 is connected to a sample-and-hold circuit 3 through a buffer 2. The sample-and-hold circuit 3 includes an electronic switch 4 and a capacitor 5. A clock pulse signal from a clock generating circuit 6 is applied to the digital-to-analog converter 1, and the sample-and-hold circuit 3 is connected to a LPF (Low-Pass Filter) 8 through the buffer 7. The output signal of the LPF 8 is the analog output of the digital-to-analog converter circuit.
In the conventional digital-to-analog converter circuit thus constructed, the digital-to-analog converter 1 first converts the applied audio digital signal into an analog signal at a sample rate determined by the applied clock pulse signal. The analog signal is supplied to the sample-and-hold circuit 3 through the buffer 2, and stable portions of the analog signal are sampled and held in response to the clock pulse signal. The frequency F.sub.1 of the clock pulse signal is over twice as high as the highest frequency F.sub.2, of the audio signal. Consequently, the frequency spectrum of the output of the buffer 7 is, as shown in FIG. 2, composed of a primary signal of the audio signal (lower than F.sub.2 in frequency) and harmonics of .+-.F.sub.2 centered around F.sub.1. As shown in FIG. 3, since the cut-off frequency is set at the level of F.sub.2 by the LPF 8, the frequency spectrum of the output of the LPF 8 is composed of only the primary signal component of the audio signal, as shown in FIG. 4. In digital audio disc players, the frequency F.sub.2 is normally set at approximately 20 KHz.
However, the LPF 8 in the conventional digital-to-analog converter circuit requires steep cut-off characteristics, making the circuit of the LPF 8 costly. Also, the presence of the LPF 8 causes changes to the phase characterisitics within the audio range, reducing the overall sound quality. In addition to the above problems, noise is generated in the LPF, even if a high quality LPF is used, so that the dynamic range of the player may be adversely affected.